Microparticles and nanoparticles can be formed using a variety of processes which vary depending on the materials and desired sizes. Mechanical milling, attritors, deposition methods, and chemical decomposition methods have all been used with varying degrees of success to produce particles of various materials. There are currently three primary competing approaches. For materials like aluminum, where the vaporization temperature is not too high, or where suitable gas-phase organometallic precursors exist, it is possible to thermally or by reactions, generate metal atoms in the gas phase, and to grow nanoparticles by controlled aggregation. The disadvantages of this approach are that the energy cost (vaporization) or reagent costs (organometallics) are high, and once the particles are generated they must separately be passivated to prevent aggregation and/or ignition upon exposure to air. Similarly, it is possible to generate nanoparticles in solution phase by decomposing precursors. These particles are typically made with a surfactant that adds solubility and protects the particles. The disadvantage is that the precursors are expensive, so that the method is not suitable for high volume application. Finally, ball milling is often used to reduce micron size powders to the nanoscale by milling the micron size powder with balls made from a heavy, hard material that crushes the micron scale powder. This method is effective for hard materials, although even for such materials there are problems with aggregation and cold-welding the particles. Cold welding refers to interaction of the surfaces of nascent nanoparticles with each other, under milling conditions, that welds the particles together, making an aggregate that is difficult or impossible to disrupt. The cold-welding problem is particularly severe for soft materials, and in addition, these materials may not mill efficiently because they are too ductile, such that the micron size particles simply deform under impact of the balls, rather than shattering. For example, if 20 micron aluminum particles are milled with stainless steel balls the average size is reduced to ˜800 nm, but additional milling fails to cause any further size reduction. Furthermore, we have data showing that if we start the conventional milling process with 50 nm aluminum particles, the average particle size increases with milling (to ˜1100 nm), due to cold welding.
Mechanical milling, such as ball milling, is routinely used in a wide variety of applications and involves the use of a milling media (e.g. steel or tungsten carbide balls) and a feedstock. The milling media and feedstock are vigorously mixed such that the feedstock is crushed into smaller pieces to form the desired particulate material. Although smaller particles can generally be achieved by longer milling times, there is a limit to this effect which is tempered by agglomeration of particles and mechanical limitations. Agglomeration of particles is particularly acute for ductile materials which tend to cold weld. As such, convenient and improved methods to produce micron and nanoparticles of such materials continue to be sought.